Apparatus of maximum likelihood signal detection

ABSTRACT

An apparatus for maximum likelihood signal detection comprises a reference data unit, a branch metric unit, an add-compare-select unit, a path metric unit and a path memory, and is used for detecting maximum likelihood signal. The reference information unit, the branch metric unit, the add-compare-select unit or the path metric unit can further comprise a multiplexer to removing unnecessary paths according to a control signal. The control signal is adjusted according to channel response, coding constraint or channel memory length.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an apparatus of maximum likelihood signaldetection, and more particularly to an apparatus that realizes maximumlikelihood detection for detecting signals with differentcharacteristics. The apparatus of maximum likelihood signal detectioncan be used to detect signals with different channel responses, codingconstraints and channel memory lengths.

2. Description of Related Art

A conventional apparatus of maximum likelihood signal detection makesuse of the Viterbit algorithm for signal detection or decoding, and isthus also called a Viterbit decoder or a Viterbit detector. Theconventional Viterbit algorithm is usually used in communication systemsor data storage systems, and can be used for decoding of convolutionalcodes, detection of baseband signals of wireless communication systemsor detection of data recorded on a harddisk.

With gradual popularity of optical discs in recent years, the demand ofincreasing the capacity of optical discs rises more and more. In orderto increase the data capacity of optical discs, many specifications ofoptical discs have been worked out, e.g., VCD, DVD, HDDVD and Blue-raydisc. However, because optical discs generally record data with landsand pits, in order to meet the demand of increasing capacities ofoptical discs, the formed lands and pits will be denser. Hence it makesreading of the data from the optical disk becoming more difficult.Therefore, in orders to conquer the problem of data reading,manufacturers utilize partial response maximum likelihood (PRML) deviceswith the Viterbit decoder for data detection.

FIG. 1 is a block diagram of a conventional Viterbit decoder applied ina partial response maximum likelihood device. As shown in FIG. 1, aViterbit decoder 10 comprises a branch metric unit 101, anadd-compare-select unit 103, a path metric unit 105, a path memory 107and a reference information unit 109.

The branch metric unit 101 is used for receiving reference informationoutputted by the reference information unit 109 and a sampled datasignal to calculate out a first calculation value and also foroutputting the first calculation value to the add-compare-select unit103. The reference information is a reference signal level of a branch.The first calculation value is a branch metric of the branch. Theadd-compare-select unit 103 performs addition, comparison and selectionactions to generate a second calculation value (i.e., a survival pathmetric), and transmits the survival path metric to the path metric unit105 to update the path metric stored in the path metric unit 105.

The add-compare-select unit 103 will retrieve the latest path metricfrom the path metric unit 105 to perform addition, comparison andselection actions so as to generate the next second calculation value.Besides, the add-compare-select unit 103 will also transmit itsdetermination result (i.e., a determination bit) to the path memory 107to temporarily store the survival path into the path memory 107. Next,the path memory 107 will output the path data to a next-stage device forsubsequent data processing. Generally, the addition, comparison andselection actions of the add-compare-select unit 103 are carried out inturn. But some methods, however, the addition action and comparisonaction are performed parallel to enhance the operational speed. Althoughvarious embodiments of the invention are exemplified with theadd-compare-select unit, the invention also applies to these kinds ofmethods with faster operational speeds.

Data signals reproduced from various optical discs, however, may havedifferent characteristics, for example, different channel responses,different coding constraints or different channel memory lengths. In theprior art, various maximum likelihood signal detection apparatuses aretherefore used for respectively processing the received data signalsfrom those optical discs, hence easily resulting in waste of resourcesand increase of the manufacturing cost.

SUMMARY OF THE INVENTION

The primary object of the invention is to provide an apparatus formaximum likelihood signal detection, which is used to realize maximumlikelihood detection to detect signals with different characteristics.The apparatus of maximum likelihood signal detection can be used todetect signals with different channel responses, coding constraints andchannel memory lengths.

Another object of the invention is to provide an apparatus of maximumlikelihood signal detection, which is used to realize maximum likelihooddetection and can read two-bit data at a time by using a trellis diagramwith a number of read bit M=2.

In order to achieve the above objects, the invention provides anapparatus of maximum likelihood signal detection, which comprises areference information unit for generating a reference signal, a branchmetric unit for receiving an input signal and the referencesignalreference information unit to generate a first branch metric, anadd-compare-select unit for receiving the first branch metric andperforming addition, comparison and selection actions to generate asurvival path metric and a determination bit, a path metric unit forstoring the survival path metric, and a path memory for storing thedetermination bit generated by the add-compare-select unit.

The branch metric unit comprises a branch meter for generating a secondbranch metric according to the input signal and the reference signal,and comprises a multiplexer for outputting the second branch metric or apredetermined branch metric to be the first branch metric according to acontrol signal. The control signal can be adjusted according to thecorresponding condition of the input signal. The corresponding conditioncomprises channel response, coding constraint or channel memory length.

To achieve the above objects, the invention also provides an apparatusfor maximum likelihood signal detection, which comprises a referenceinformation unit for generating a first reference signal, a branchmetric unit for receiving an input signal and the first referencesignalreference information unit to generate a branch metric, anadd-compare-select unit for receiving the branch metric and performingaddition, comparison and selection actions to generate a survival pathmetric and a determination bit, a path metric unit for storing thesurvival path metric, and a path memory for storing the determinationbit.

The reference information unit comprises a reference signal generatorfor generating a second reference signal according to the input signal,and a multiplexer for outputting the second reference signal or apredetermined reference signal to be the first reference signalaccording to a control signal. The control signal can be adjustedaccording to the corresponding condition of the input signal. Thecorresponding condition comprises channel response, coding constraint orchannel memory length.

In order to achieve the above objects, the invention also provides anapparatus for maximum likelihood signal detection, which comprises areference information unit for generating a reference signal, a branchmetric unit for receiving an input signal and the referencesignalreference information unit to generate a first branch metric, anadd-compare-select unit receiving a second branch metric and performingaddition, comparison and selection actions to generate a survival pathmetric and a determination bit, a path metric unit for storing thesurvival path metric, and a path memory for storing the determinationbit.

The add-compare-select unit further comprises a multiplexer foroutputting the first branch metric or a predetermined input valueaccording to a control signal. The control signal can be adjustedaccording to the corresponding condition of the input signal. Thecorresponding condition comprises channel response, coding constraint orchannel memory length.

In order to achieve the above objects, the invention also provides anapparatus for maximum likelihood signal detection, which comprises areference information unit for generating a reference signal, a branchmetric unit for receiving an input signal and the referencesignalreference information unit to generate a branch metric, anadd-compare-select unit for receiving the branch metric and performingaddition, comparison and selection actions to generate a survival pathmetric and a determination bit, a path metric unit for storing thesurvival path metric, and a path memory for storing the determinationbit.

The path metric unit comprises a path metric memory for storing thesurvival path metric, and a multiplexer for outputting the stored pathmetric or a predetermined path metric to the add-compare-select unitaccording to a control signal. The control signal can be adjustedaccording to the corresponding condition of the input signal. Thecorresponding condition comprises channel response, coding constraint orchannel memory length.

BRIEF DESCRIPTION OF THE DRAWINGS

The various objects and advantages of the invention will be more readilyunderstood from the following detailed description when read inconjunction with the appended drawing, in which:

FIG. 1 is a block diagram of a conventional Viterbit decoder applied ina partial response maximum likelihood device;

FIG. 2 shows trellis diagrams of the invention with a channel memorylength n=3 and a coding constraint d=0, 1, 2, and 1/2, respectively;

FIG. 3 shows trellis diagrams of the invention with a channel memorylength n=4 and a coding constraint d=0, 1, 2, and 1/2, respectively;

FIG. 4 shows trellis diagrams of the invention with a channel memorylength n=3, 4 and 3/4 and a coding constraint d=1/2, respectively;

FIG. 5 shows a trellis diagram of the invention with a channel memorylength n=3/4 and a coding constraint d=1/2;

FIG. 6A is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a first embodiment of the invention;

FIG. 6B is a more detailed system block diagram for an apparatus ofmaximum likelihood signal detection according to the first embodiment ofthe invention;

FIG. 7 is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a second embodiment of the invention;

FIG. 8 is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a third embodiment of the invention;

FIG. 9A is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a fourth embodiment of the invention;

FIG. 9B is another system block diagram of an apparatus for maximumlikelihood signal detection according to the fourth embodiment of theinvention;

FIGS. 10A and 10B are trellis diagrams of the invention with a channelmemory length n=3, a coding constraint d=1/2 and a number of read bitM=2;

FIGS. 11A and 11B are trellis diagrams of the invention with a channelmemory length n=4, a coding constraint d=1/2 and a number of read bitM=2;

FIG. 12 is a trellis diagram of the invention with a channel memorylength n=3/4, a coding constraint d=1/2 and a number of read bit M=2;

FIG. 13 is another trellis diagram of the invention with a channelmemory length n=3/4, a coding constraint d=1/2 and a number of read bitM=2; and

FIG. 14 is yet another trellis diagram of the invention with a channelmemory length n=3/4, a coding constraint d=1/2 and a number of read bitM=2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention discloses an apparatus for maximum likelihood signaldetection, which detects signals with various channel responses, codingconstraints and/or channel memory lengths read from optical discs ofdifferent specifications according to the maximum likelihood detectiontheory. The invention combines various trellis diagrams of varioussignals read from optical discs of different specifications to lower thecomplexity of hardware or software system.

FIG. 2 shows trellis diagrams of the invention with a channel memorylength n=3 and a coding constraint d=0, 1, 2, or 1/2, respectively. Thechannel memory length is used to describe the size of a laser light spotprovided by the pickup head on the optical disc. A channel memory lengthof n=3 means the size of the spot covering three bits data on the disk.

The coding constraint is also called a run length limited (RLL), and isused to limit the number of consecutive “0” (i.e., pits) or “1” (i.e.,land) in the data string so as to facilitate discrimination of the data.A coding constraint of d=0 means the data can be arranged freely, acoding constraint of d=1 means the data must have at least twoconsecutive “0” or “1”, a coding constraint of d=2 means the data musthave at least three consecutive “0” or “1”. It should be noted that thetrellis diagram with a coding constraint d=1/2 in FIG. 2 illustrate thecombination of the trellis diagrams with a coding constraint d=1 andd=2.

Using the trellis diagram with a channel memory length n=3 and a codingconstraint d=0 in FIG. 2 as an example, the left side shows the possiblestates at a first time point, while the right side shows the possiblestates at a second time point. Moving the light spot from the positionat the first time point to the position at the second time pointrepresents the light spot shifts a one-bit length on the optical disc.The coding constraint d=0 in this trellis diagram means that the datacan be arranged freely. Therefore, each state at the first time pointcan generate two branches respectively pointing toward the states at thesecond time point.

Using the trellis diagram with a channel memory length n=3 and a codingconstraint d=1 in FIG. 2 as an example, the coding constraint d=1 inthis trellis diagram represents that the data must have at least twoconsecutive “0” or “1”. Therefore, both the states of first time pointand the states of second time point of this trellis diagram have twonon-existent data states (i.e., data states having no branchconnections).

Similarly, using the trellis diagram with a channel memory length n=3and a coding constraint d=2 in FIG. 2 as an example, the codingconstraint d=2 in this trellis diagram represents the data must have atleast three consecutive “0” or “1”. Therefore, both the states of firsttime point and the states of second time point of this trellis diagramhave two non-existent data states. Besides, because the codingconstraint d=2 is more stringent than the coding constraint d=1, hence,the trellis diagram with a coding constraint d=2 has less branches.Moreover, because the trellis diagram of the coding constraint d=2conforms to the trellis diagram of coding constraint d=1. In the otherword, the branches existing in the trellis diagram with a codingconstraint d=2 will surely exist in the trellis diagram with a codingconstraint d=1.

In consideration of the above statements, the invention combines thesetwo trellis diagrams with different coding constraints d=1 and d=2 intoa trellis diagram with a coding constraint d=1/2. In the trellis diagramwith a coding constraint d=1/2, most of the branches are common branchesfor the trellis diagram with a coding constraint d=2 and the trellisdiagram with a coding constraint d=1. There are only two differentbranches marked by circle symbols in FIG. 2 which is used only for thetrellis diagram with a coding constraint d=1. Therefore, by applying ormasking these two branches, the trellis diagram with a coding constraintd=1/2 can be converted to the trellis diagram with a coding constraintd=1 or the trellis diagram with a coding constraint d=2.

FIG. 3 shows trellis diagrams of the invention with a channel memorylength n=4 and a coding constraint d=0, 1, 2, and 1/2, respectively.FIG. 3 is quite similar to FIG. 2. They only differ in that there aremore data states and thus more complex branch connections in FIG. 3. Inthe trellis diagram with a coding constraint d=0, because the data canbe arranged freely, the data state covered at each first time point cangenerate two branches respectively pointing toward the data statecovered at the second time point.

In the trellis diagram with a coding constraint d=1 in FIG. 3, thecoding constraint d=1 means that the data must have at least twoconsecutive “0” or “1”. Therefore, both the states of first time pointand the states of second time point in this trellis diagram have sixnon-existent data states (i.e., data states having no branchconnections). Similarly, in the trellis diagram with a coding constraintd=2, the coding constraint d=2 means the data must have at least threeconsecutive “0” or “1”. Therefore, both the states of first time pointand the states of second time point in this trellis diagram have eightnon-existent data states.

Generally, the trellis diagram can be defined in two ways. First, thetrellis diagram is defined by that the reference signal has relationshipwith the latest data. Or, the trellis diagram is defined by that thereference signal has not relationship with the latest data. These twokinds of trellis diagrams make different definitions to the channelmemory length n. No matter which kind of trellis diagram is applied, allthe embodiments of the invention can be applied. For the sake ofbrevity, only one of them will be described herein in detail.

In FIG. 3, the trellis diagrams with coding constraints d=1 and d=2 arecombined to form the trellis diagram with a coding constraint d=1/2. Inthe trellis diagram with a coding constraint d=1/2, most of the branchesare common branches for the trellis diagram with a coding constraint d=2and the trellis diagram with a coding constraint d=1. There are onlyfour different branches marked by square symbols in FIG. 3 which is usedonly for the trellis diagram with a coding constraint d=1. Therefore, byapplying or masking these four branch connections, the trellis diagramwith a coding constraint d=1/2 can be converted to the trellis diagramwith a coding constraint d=1 or the trellis diagram with a codingconstraint d=2.

Although the data encoding method for data stored on optical discs aregenerally using RLL codes. But, there are various RLL codes codingconstraint for the various optical discs with different specifications.For instance, the coding constraint is equal to 0, 1 or 2. Therefore,the conventional optical disc drives usually have to use multiplemaximum likelihood signal detection apparatuses or Viterbit detectors torestore the data recorded on an optical disc according to differenttrellis diagrams. In the invention, by using the trellis diagram with acoding constraint d=1/2 and by applying or masking different branchconnections, the apparatus for maximum likelihood signal detection ofthe invention can suitable for reading various optical discs withdifferent coding constraints (RLL codes).

In order to make the apparatus for maximum likelihood signal detectionof the invention apply to optical discs with different channel memorylengths, the invention further combine the trellis diagrams shown inFIG. 2 and FIG. 3 to form the trellis diagrams shown in FIG. 4. FIG. 4shows trellis diagrams of the invention with a channel memory lengthn=3, 4 and 3/4 and a coding constraint d=1/2, respectively. The trellisdiagram with a coding constraint d=1/2 and a channel memory length n=3/4in FIG. 4 is formed by the combination of the trellis diagrams with acoding constraint d=1/2 and a channel memory length n=3 and n=4. Inother word, the trellis diagram in FIG. 4 is formed by the trellisdiagram with a coding constraint d=1/2 and a channel memory length n=3in FIG. 2 and the trellis diagram with a coding constraint d=1/2 and achannel memory length n=4 in FIG. 3.

Specially, the left and right sides of the trellis diagram shown in FIG.4 illustrate the data states of the channel memory length n=4 by fourbits and illustrate the data states of the channel memory length n=3 bythree bits marked by underlines.

As shown in FIG. 4, if an optical disc has a channel memory length n=4and a coding constraint d=2, the branch connections marked by squaresymbols and the branch connections only for the channel memory lengthn=3 can be masked. If an optical disc has a channel memory length n=4and a coding constraint d=1, the branch connections marked by squaresymbols can be applied, while the branch connections only for thechannel memory length n=3 can be masked.

Similarly, if an optical disc has a channel memory length n=3 and acoding constraint d=2, the branch connections marked by circle symbolsand the branch connections only for the channel memory length n=4 can bemasked. If an optical disc has a channel memory length n=3 and a codingconstraint d=1, the branch connections marked by circle symbols can beapplied, while the branch connections only for the channel memory lengthn=4 can be masked.

However, the trellis diagram with a coding constraint d=1/2 and achannel memory length n=3/4 in FIG. 4 increase the number of branchconnections toward the data state at the second time point, i.e. themaximum number of branch connections toward the data state at the secondtime point become three. This will make the design of anadd-compare-select unit of the invention become more complex.

FIG. 5 shows another trellis diagram of the invention with a channelmemory length n=3/4 and a coding constraint d=1/2. Comparing to thetrellis diagram shown in FIG. 4, this trellis diagram can reduce thedesign complexity of the invention. Similarly to FIG. 4, the left andright sides of the trellis diagram in FIG. 5 illustrate the data statesof the channel memory length n=4 by four bits and illustrate the datastates of the channel memory length n=3 by three bits marked byunderlines. Because the trellis diagram in FIG. 5 decreases the maximumnumber of branch connections toward the data state at the second timepoint to two, the design complexity of the invention can be reduced.

FIG. 6A is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a first embodiment of the invention. Asshown in FIG. 6A, an apparatus of maximum likelihood signal detection 60of the invention includes a branch metric unit 601, anadd-compare-select unit 603, a path metric unit 605, a path memory 607and a reference information unit 609. The branch metric unit 601 furtherincludes a branch meter 6011 and a multiplexer (mux) 6013.

The branch metric unit 601 receives a reference signal level outputtedby the reference information unit 609 and a sampled signal read from anoptical disc to obtain a first branch metric, and then outputs the firstbranch metric to the add-compare-select unit 603. The sampled signal ofdata read from the optical disc and the reference signal level are firstsent to the branch meter 6011 to get a second branch metric, and thesecond branch metric is transmitted to the mux. 6013. The mux. 6013 thenoutputs the second branch metric or a predetermined branch metric to bethe first branch metric according to a control signal.

It should be noted that the system of the invention will set the controlsignal, i.e., adjust the control signal according to different channelresponses, coding constraints or channel memory lengths. For example, ifthe system of the invention read signals from an optical disc with achannel memory length n=3 and a coding constraint d=2, the controlsignal can drive the mux. 6013 to select the predetermined branch metricwhich will cause the add-compare-select unit 603 do not select thisbranch connection so as to mask the unnecessary branch connection (i.e.,those branch connections not belonging to the trellis diagram with achannel memory length n=3 and a coding constraint d=2). Thepredetermined branch metric can be set to, for example, a large enoughvalue or a small enough value so that the add-compare-select unit 603will not select the masked branch connections after performing theaddition and comparison actions.

The add-compare-select unit 603 performs the addition, comparison andselection actions to obtain a survival path metric and transmits thesurvival path metric to the path metric unit 605 for further updatingthe path metric stored in the path metric unit 605.

The add-compare-select unit 603 retrieves the latest path metric fromthe path metric unit 605 to perform the addition, comparison andselection actions after receiving the next branch metric and furthergenerates the next survival path metric. Besides, the add-compare-selectunit 603 also sends a determination result signal (i.e., a determinationbit) to the path memory 607 to update the survival path stored in thepath memory 607. Next, the path memory 607 will output the path datathat has been finally confirmed to a next-stage device for follow-updata processing.

FIG. 6B is a more detailed description to the system block diagram ofthe apparatus for maximum likelihood signal detection according to thefirst embodiment of the invention. As shown in FIG. 6B, the sampledsignal of data read from the optical disc and the reference signal arefirst transmitted to the branch meter 6011 to generate a second branchmetric which further includes a branch metric B_(0001→0011) and a branchmetric B_(1001→0011). The branch metric B_(1001→0011) and apredetermined branch metric are sent to the mux. 6013. The mux. 6013will output the branch metric B_(1001→0011) or the predetermined branchmetric to a add-compare-select 603 according to a control signal. Theoutput of the mux. 6013 then transfer to an adder to perform an additionaction with a survival path metric P₁₀₀₁ retrieved from the path metricunit 605, and then, transfer the addition result to thecompared-selector 6031. On the other hand, the branch metricB_(0001→0011) also added with a survival path metric P₀₀₀₁ retrievedfrom the path metric unit 605, then the addition result will also besent to the compared-selector 6031. Finally, the compared-selector 6031will generate a latest survival path metric P₀₀₁₁ and a determinationresult signal Sel₀₀₁₁. The latest survival path metric P₀₀₁₁ will betransfer to the path metric unit 605 and the determination result signalSel₀₀₁₁ will be transfer to the path memory 607 to update the survivalpath stored in the path memory 607.

FIG. 7 is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a second embodiment of the invention. Asshown in FIG. 7, an apparatus for maximum likelihood signal detection ofthe invention 70 includes a branch metric unit 701, anadd-compare-select unit 703, a path metric unit 705, a path memory 707and a reference information unit 709. The reference information unit 709further includes a reference signal generator 7091 and a multiplexer(mux) 7093.

The branch metric unit 701 receives a first reference signal output bythe reference information unit 709 and sampled signal of data read froman optical disc to obtain a branch metric, and outputs the branch metricto the add-compare-select unit 703. The reference signal generator 7091of the reference information unit 709 generate a second reference signalaccording to the sampled signal of data read from the optical disc andtransmit the second reference signal to the mux. 7093. The mux. 7093then selectively outputs the second reference signal or a predeterminedreference signal to be the first reference signal according to a controlsignal.

It should be noted that the system of the invention will set the controlsignal, i.e., adjust the control signal according to different channelresponses, coding constraints or channel memory lengths. For example, ifthe system of the invention reads data of the optical disc with achannel memory length n=3 and a coding constraint d=2, the controlsignal can drive the mux. 7093 to select the predetermined referencesignal which will cause the add-compare-select unit 703 do not selectthis branch connection so as to mask the unnecessary branch connections(i.e., those branch connections not belonging to the trellis diagramwith a channel memory length n=3 and a coding constraint d=2). Thepredetermined reference signal can be set to, for example, a largeenough (or small enough) value so that the branch metric unit 701 willgenerate a large enough branch metric and therefore theadd-compare-select unit 703 will not select the masked branchconnections after performing the addition and comparison actions.

After the add-compare-select unit 703 receives the branch metric, itwill perform the addition, comparison and selection actions to generatea survival path metric and transmit the survival path metric to the pathmetric unit 705 so as to update the path metric stored in the pathmetric unit 705.

After the add-compare-select unit 703 receives the next branch metric,it will retrieve the latest path metric from the path metric unit 705 toperform the addition, comparison and selection actions so as to generatethe next survival path metric. Besides, the add-compare-select unit 703will also send its determination result signal (i.e., a determinationbit) to the path memory 707 to update the survival path stored in thepath memory 707. Next, the path memory 707 will output the path datathat has been finally confirmed to a next-stage device for follow-updata processing.

FIG. 8 is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a third embodiment of the invention. Asshown in FIG. 8, an apparatus for maximum likelihood signal detection ofthe invention 80 comprises a branch metric unit 801, anadd-compare-select unit 803, a path metric unit 805, a path memory 807and a reference information unit 809. The add-compare-select unit 803further includes an adder-comparator-selector 8031 and a mux. 8033.

The branch metric unit 801 receives a reference signal outputted by thereference information unit 809 and a sampled signal corresponding todata read from an optical disc to obtain a branch metric, and outputs abranch metric to the mux. 8033 in the add-compare-select unit 803. Themux. 8033 outputs the branch metric or a predetermined input value tothe adder-comparator-selector 8031 according to a control signal.

It should be noted that the system of the invention will set the controlsignal, i.e., adjust the control signal according to different channelresponses, coding constraints or channel memory lengths. For example, ifthe system of the invention reads data of the optical disc with achannel memory length n=3 and a coding constraint d=2, the controlsignal can drive the mux. 8033 to output the predetermined input valuewhich will cause the adder-comparator-selector 8031 do not select thisbranch connection so as to mask the unnecessary branch connections(i.e., those branch connections not belonging to the trellis diagramwith a channel memory length n=3 and a coding constraint d=2). Thepredetermined input value can be set to, for example, a large enoughvalue or a small enough value so that the adder-comparator-selector 8031will not select the masked branch connections after performing theaddition and comparison actions.

Next, the add-compare-select unit 803 will generate a survival pathmetric and transmit the survival path metric to the path metric unit 805so as to update the path metric stored in the path metric unit 805.Besides, the add-compare-select unit 803 will also send a determinationresult signal (i.e., a determination bit) to the path memory 807 toupdate the survival path stored in the path memory 807. Next, the pathmemory 807 will output the path data that has been finally confirmed toa next-stage device for follow-up data processing.

FIG. 9A is a system block diagram of an apparatus for maximum likelihoodsignal detection according to a fourth embodiment of the invention. Asshown in FIG. 9A, an apparatus for maximum likelihood signal detectionof the invention 90 comprises a branch metric unit 901, anadd-compare-select unit 903, a path metric unit 905, a path memory 907and a reference information unit 909. The path metric unit 905 furthercomprises a path metric memory 9051 and a multiplexer (mux) 9053.

The branch metric unit 901 receives a reference signal outputted by thereference information unit 909 and a sampled signal corresponding todata read from an optical disc to obtain a branch metric, and outputsthe branch metric to the add-compare-select unit 903. The referenceinformation unit 909 will calculate out a corresponding reference signalaccording to the sampled signal of data read from the optical disc.

After the add-compare-select unit 903 receives the branch metric, itwill perform the addition, comparison and selection actions to generatea survival path metric and transmit the survival path metric to the pathmetric unit 905 so as to update the path metric stored in the pathmetric memory 9051 of the path metric unit 905. After theadd-compare-select unit 903 receives the next branch metric, it willretrieve the latest path metric from the path metric unit 905 to performthe addition, comparison and selection actions so as to generate thenext survival path metric. Besides, the add-compare-select unit 903 willalso send its determination result signal (i.e., a determination bit) tothe path memory 907 to update the survival path stored in the pathmemory 907. Next, the path memory 907 will output the path data that hasbeen finally confirmed to a next-stage device for follow-up dataprocessing.

The mux. 9053 of the path metric unit 905 determines to output the pathmetric provided by the path metric memory 9051 or a predetermined pathmetric according to a control signal. It should be noted that the systemof the invention will set the control signal, i.e., adjust the controlsignal according to different channel responses, coding constraints orchannel memory lengths. For example, if the system of the inventionreads data of the optical disc with a channel memory length n=3 and acoding constraint d=2, the control signal can drive the mux. 9053 toselect the predetermined path metric which will cause theadd-compare-select unit 903 do not select this branch connection so asto mask the unnecessary branch connections (i.e., those branchconnections not belonging to the trellis diagram with a channel memorylength n=3 and a coding constraint d=2). The predetermined path metriccan be set to, for example, a large enough value or a small enough valueso that the add-compare-select unit 903 will not select the maskedbranch connections after performing the addition and comparison actions.

The embodiment in FIG. 9A selectively changes the path metric accordingto the control signal by using the mux. As shown in FIG. 9B, this canalso be accomplished by directly update the stored path metric in thepath metric memory 9051 to be the predetermined path metric according tothe control signal. FIG. 9B is another system block diagram of anapparatus for maximum likelihood signal detection according to the fifthembodiment of the invention. It should be noted that the system of theinvention will set the control signal, i.e., adjust the control signalaccording to different channel responses, coding constraints or channelmemory lengths.

In order to enhance the data reading efficiency of the optical system,the invention further combines trellis diagrams generated by the readingsignal of the optical disc so that the apparatus for maximum likelihoodsignal detection of the invention can read two-bit data each time. FIGS.10A and 10B are trellis diagrams of the invention with a channel memorylength n=3, a coding constraint d=1/2 and a number of read bit M=2. Asshown in FIG. 10A, when the apparatus for maximum likelihood signaldetection of the invention carries out reading of two-bit data, it hasto perform the data determination twice time. In order to enhance theefficiency of data reading, however, the invention provides trellisdiagrams with a number of read bit M=2 shown in FIG. 10B. According tothe trellis diagram shown in FIG. 10B, the apparatus for maximumlikelihood signal detection of the invention requires only one time ofthe data determination for reading of two-bit data. Similarly, thetrellis diagrams with a channel memory length n=4, coding constraintsd=1/2 and a number of read bit M=2 can be provided as shown in FIGS. 11Aand 11B.

Of course, the invention can further be extended to an apparatus formaximum likelihood signal detection capable of reading three or more-bitdata each time. Therefore, the invention is not limited to the aboveembodiments, and can be designed according to the practical necessity ofthe user.

In order to make the apparatus for maximum likelihood signal detectionof the invention be able to read two-bit data simultaneously and applyto optical discs with different channel memory lengths, the trellisdiagrams in FIGS. 10B and 11B are further combined, as shown in FIGS.12, 13 and 14. FIGS. 12 and 13 provide a preferred combining mannerbecause their maximum number of branch connections toward the data stateof the second time point is equal to 4, while that of FIG. 14 is equalto 5.

The architecture of the first to fifth embodiments can generally be usedfor data reading actions of the trellis diagram shown in FIGS. 12, 13and 14. The difference is primarily the number of branch connectionstoward the data state at the second time point. Therefore, theadd-compare-select unit has to compare three or more path metrics fordetermining two-bit data.

Although the invention has been described with reference to thepreferred embodiment thereof, it will be understood that the inventionis not limited to the details thereof. Various substitutions andmodifications have been suggested in the foregoing description, andother will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the invention as defined in the appended claims.

1. An apparatus for maximum likelihood signal detection comprising: areference information unit for generating a reference signal; a branchmetric unit for receiving an input signal and the reference signal togenerate a first branch metric, wherein the branch metric unitcomprises: a branch meter for generating a second branch metricaccording to the input signal and the reference signal; and amultiplexer for outputting the second branch metric or a predeterminedbranch metric to be the first branch metric according to a controlsignal; an add-compare-select unit for receiving the first branch metricand performing addition, comparison and selection actions to generate asurvival path metric and a determination result signal; a path metricunit for storing the survival path metric; and a path memory for storinga path data according to the determination result signal.
 2. Theapparatus for maximum likelihood signal detection as claimed in claim 1,wherein the control signal is adjusted according to the correspondingcondition of the input signal, wherein the corresponding conditioncomprises channel response, coding constraint or channel memory lengthof the input signal.
 3. The apparatus for maximum likelihood signaldetection as claimed in claim 2, wherein the coding constraint is equalto 1 or
 2. 4. The apparatus for maximum likelihood signal detection asclaimed in claim 2, wherein the channel memory length is equal to 3 or4.